A representative diesel engine comprises an exhaust system through which diesel exhaust gas is conveyed from the engine's cylinders to the tailpipe where it enters the surrounding atmosphere. Before passing through and out of the tailpipe, the exhaust gas is treated by an after-treatment system that comprises one or more after-treatment devices. Examples of such devices are diesel oxidation catalysts (DOC's) and diesel particulate filters (DPF's).
Diesel engines that are manufactured today for use in automotive vehicles such as trucks are typically turbocharged. Heat energy in exhaust gas leaving an engine exhaust manifold is converted into mechanical energy as it passes through a turbine portion of a turbocharger to operate a compressor in the compression portion that draws air into the intake system to deliver charge air to the engine cylinders.
For controlling NOx content in the exhaust gas, a portion of the exhaust gas can be recirculated from the exhaust system through an EGR system to the intake system. When the entrance to an EGR system is upstream of the turbocharger turbine, it may be appropriate for the EGR system to comprise one or more heat exchangers for cooling the exhaust gas being recirculated. Cooling of the exhaust gas can increase the effectiveness of the EGR system in limiting the generation of NOx.
However, it is recognized in the industry that cooling of recirculated exhaust gas creates the potential for condensation of certain gaseous constituents in the exhaust gas. Over time such condensates may accumulate sufficiently to have a detrimental effect on performance and/or components. For example, coolant passageways in coolers may become restricted, components may corrode, and moving parts may stick.
Condensation may be more extreme and/or perhaps even unavoidable at certain times, such as when a cold-soaked engine is warming after having been started and portions of its EGR system have not yet reached operating temperature. When condensation occurs along an EGR flow path and temperature of surrounding parts is sufficiently low, condensate may freeze and consequently restrict, or even block, the flow until the parts warm sufficiently to thaw the frozen condensate.
Condensation may also occur regardless of whether any cooler is present in the EGR system, for example when the engine is running at low idle and exhaust gas temperatures are low.
To address such situations, known practices in EGR control strategies include delaying and/or limiting EGR when conditions are conducive for condensation, and while such measures may be helpful in slowing the accumulation of condensates as an engine ages, they do impact the quantity of NOx in tailpipe emissions.